Lettieri was another of those brilliant casting choices for GF and II. Although Sollozzo is a pivotal character in terms of what he sets in motion, he was limited to a relatively small screen presence. But Lettieri was so letter-perfect for that role that he's unforgettable. One of Lettieri's assets was that, like Lee Marvin, he combined a coarse, often menacing, appearance with a cultured voice that was alternately smooth and then snarling.
Turnbull, I think you hit upon one of the great factors in brilliancy of the Godfather films. Picking the right actor for the right character. As I previously stated many of the secondary characters were top actors, who usually got more screen time in past films. I really can't think of a weak character in either GFI or II. Mix in FFC attention to detail, and you have the ingredients for a great film.
